The present invention relates to a method for driving and simultaneously evaluating a rate-of-turn sensor and a circuit design for carrying out the inventive method.
Micromechanical rate-of-turn sensors are used, e.g., in motor vehicles for the functionality of the electronic stability program, ESP, or for roll-tendency compensation. They use the Coriolis effect to measure, e.g., the rate-of-turn around the vertical axis or longitudinal axis of the motor vehicle.
Micromechanical rate-of-turn sensors contain one or more elastically suspended oscillator elements, which are stimulated to perform a periodic oscillating motion using driving forces F(t) that change periodically over time and are applied electrostatically. Simply speaking, the oscillator elements are elastically suspended masses m.
The plane in which the flat oscillator element lies is the x-y plane. When mass m that is swinging in the x direction is rotated around the z axis—which is normal to the x-y plane and which can be, e.g., the vertical axis of the vehicle—at a rate of turn Ω, mass m undergoes additional periodic acceleration in the y direction, which is proportional to rate of turn Ω. This acceleration is known as Coriolis acceleration. Special measuring devices are required to measure this acceleration, e.g., a second mass mc, which is elastically coupled to a mass that is the oscillator element and which can oscillate in the y direction, and, e.g., two precision capacitor groups for measuring the course of oscillation in the y direction. This second mass is referred to below as Coriolis mass mc. Simply speaking, Coriolis mass mc is a Coriolis element that is elastically located on the oscillator element. The oscillator element can undergo oscillations along a first axis of oscillation that represents the x axis; the Coriolis element can under oscillations along a second axis of oscillation that is normal to the first axis of oscillation and represents the y axis.
Sensors of this type are operated at the mechanical resonant frequency for the oscillator element that includes mass m, and for the Coriolis element that includes Coriolis mass mc. To accomplish this, a suitable excitation frequency of driving force F(t) must be selected and/or controlled. At the resonant frequency, there is no phase shift between the rate of motion v(t) of mass m induced by driving force F(t), and driving force F(t). Likewise, in the resonance state, there is no phase shift between Coriolis velocity vc(t) of Coriolis mass mc, and driving force F(t).
The motion of Coriolis mass mc can be evaluated directly using an “open loop” circuit design, or using a force negative feedback loop, which is also known as a closed loop. With force negative feedback, a controller ensures—via an electrostatic compensation force Fc(t) that is also applied to Coriolis mass mc—that Coriolis mass mc does not oscillate in the direction of the y axis and remains at rest in this direction, even when there is a rate of turn Ω. In this case, force Fc(t) to be applied is a measure of rate of turn Ω.
A force negative feedback loop has the advantage that the evaluation bandwidth can be adjusted via the controller parameters, and errors resulting from non-linearities in the sensor, e.g., non-linear springs on Coriolis mass mc, are greatly reduced.
FIG. 1 shows a block diagram with a rate-of-turn sensor DRS and a realization of force negative feedback according to the related art. The VCO/NCO (voltage/numerical controlled oscillator) block includes an oscillator that delivers the sinusoidal drive signal to generate driving force F(t) for the oscillator element with mass m. The frequency of the drive signal and, therefore, driving force F(t), is held at the resonant frequency of the oscillator element using a not-shown controller, and the amplitude of F(t) is stabilized by an amplitude regulator that regulates the amplitude of the drive signal. Rate-of-turn sensors with force negative feedback are known, e.g., from DE 102 37 410 A1 and DE 102 37 411 A1.
When the force negative feedback is realized such that the measured Coriolis velocity vc of Coriolis mass mc is provided via a control circuit at force input Fc(t) on rate-of-turn sensor DRS (FIG. 1), problems arise in practical application. The controller must not have phase rotation at the resonant frequency. The controller must suppress interferences above and below the resonant frequency. These requirements can only be met, e.g., by using a bandpass of the second or higher order as the controller. To accomplish this, however, the bandpass must have its mid-frequency exactly at the oscillator resonant frequency. Since the oscillator resonant frequencies of the sensors are lot-dependent and sample-dependent, however, each bandpass would have to be calibrated individually, which is costly.